1. Field of the Invention
The present invention relates to a method for disassembling a boiler. The present invention also relates to a method for disassembling a boiler and a boiler supporting structure, and more specifically, to a method for disassembling a large boiler and a boiler supporting structure which supports the boiler by a so-called top support method to be used in a thermoelectric power plant.
2. Discussion of the Related Art
A boiler is used, for instance, in a thermoelectric power plant, for generating steam at a high temperature and at a high pressure. The steam generated from the boiler based on a natural circulation or forced circulation system is used for obtaining energy for rotating a power-generating turbine, or the like. FIG. 19 is a diagram for explaining a large boiler 10 for use in a thermoelectric power plant. The boiler 10, which usually has a weight of more than 1000 tons, is installed by means of so-called “top support method”. In top support method, a top part of the boiler 10 is supported by a large boiler building (shed) 12 substantially constructed by a steel frame including a main girder 12c, columns 12a vertically extending from the main girder, and beams 12b (12b-1 to 12b-9). The boiler 10 is retained in the boiler building 12 by being suspended from the main girder 12c. Namely, the boiler 10 is suspended from the boiler building 12 via a plurality (for example, 20 to 100) of suspension members 14. One end of the suspension members 14 connected to the main girder 12c and another end thereof is connected to a top part 10a of the boiler 10.
Generally speaking, the boiler 10 comprises a furnace unit 20 and a heat recovery area (rear heat-exchanging unit) 22. The furnace unit 20 can be a hollow structure, for instance, multi-sided hollow structure. In the furnace unit 20, fuel is burnt by ignition burners (not shown) provided on the lateral wall of the furnace unit 20. Therefore, combustion gas is generated. At the bottom part of the furnace unit 20, a hopper part 23 with a tapered wall is provided. The hopper part 23 extends from the lower end of the furnace unit 20 with the diameter being decreased downwardly. By this configuration, the hopper part 23 collects discharged substances such as ash for easily disposing the substances.
The heat recovery area 22 is provided on a lateral side of the furnace unit 20, and an upper part of the heat recovery area 22 communicates with an upper part of the furnace unit 20. The heat recovery area 22 also has a hollow tubular configuration and the vertical length of the heat recovery area 22 is shorter than that of the furnace unit 20. Namely, the lower end of the heat recovery area 22 is positioned higher than that of the furnace unit 20 in the boiler 10. Furthermore, a plurality of superheaters 24 (24-1 to 24-5), shown by a dash-dotted line, are contained in the hollow interior of the heat recovery area 22, for superheating steam.
The combustion gas generated in the furnace unit 20 flows through a route shown by arrows 200 and 202. The heat of the combustion gas is subjected to a heat exchange in the superheaters 24, so as to rotate a power-generating turbine, to produce electrical energy. The combustion gas after the heat exchange process by the superheaters 24, that is, gas having a decreased temperature passes through a gas duct 26 (shown by the long dashed double-short dashed lines) and then to an electrical precipitator (not shown). As the superheater 24, it is possible to use a superheater or economizer, including therein a pipe for carrying water or steam therethrough.
FIG. 20 is a diagram for explaining a structure of a furnace wall 27 for the furnace unit 20. The furnace wall 27, which is a part of a boiler wall, includes an outer casing 27a and a fire resistant material 27b provided on an inner surface of the outer casing 27a (corresponding to the inner periphery of the furnace unit 20). The outer casing 27a is made of a metal, and the fire resistant material 27b is made of a fire-resistant material. The fire-resistant material often includes asbestos. Further, the fire-resistant material can be replaced by an insulating material for thermal control which is cheaper than the fire-resistant material. On an inner side of the fire resistant material 27b, heat exchange pipes 27c are provided for transporting a liquid or steam therein. By the provision of the heat exchange pipes 16, a heat exchange operation is carried out also on the furnace wall 27. Moreover, it is possible that an inner casing made of a metal is further provided on a fire resistant material 27b on the opposite side with respect to the outer casing 27a. Further, the heat recovery area 22 frequently includes a heat recovery wall made of a fire resistant material.
In the above described boiler system, it is sometimes necessary to disassemble the boiler and the additional facilities, because of the deterioration, increase of maintenance fee by the deterioration, or the lowered energy conversion efficiency. However, large boiler 10 to be disassembled has a height of about 25 m to 60 m, and is suspended from the boiler buildings 12. Therefore, disassembling operation with respect to the boiler 10 and the boiler building 12 is more difficult, comparing to the disassembling operation with respect to other buildings directly provided on the ground. Accordingly, several disassembling methods have been proposed.
For example, Japanese Kokai Publication 11(2001)-270154 discloses a method for dismantling a boiler and a boiler shed, wherein the boiler is dismantled from the lower part, and the boiler shed is dismantled from the upper part. More precisely, the dismantling method in the publication comprises five steps. In the first step A, jacks are provided on both ends of a beam (top girder) which suspends the boiler (FIG. 2 in the publication). In the subsequent step B, hanging members such as wires, which extend from the jacks, are hooked on the top girder (FIG. 4 in the publication). Then, the top girder is cut from the boiler shed in step C so that parts of the top girder which support the jacks remains on the shed and other part of the top girder which the boiler is suspended from is separated from the shed (FIG. 6 of the publication). Accordingly, the boiler is suspended from the jacks provided on the boiler shed via the top girder and the hanging members of the jacks. In the following steps D and E, the boiler supported by the top girder is lowered by the jacks, and the boiler is cut from the bottom thereof (FIGS. 7 and 8 in the publication). In the dismantling method, the boiler shed is disassembled from the top in the following step F, after completing the above steps A to E for disassembling of the boiler.
In addition to the above, another method for disassembling a boiler is disclosed in Japanese Kokai Publication 2003-301617. In this method, the boiler is supported by an ascent-descent stage which is suspended by jacks provided on the top of the boiler shed. Here, the boiler is cut from the lower part to give cut parts in the form of blocks, and the cut parts are transferred by the ascent-descent frame, to a disassembling field prepared on the ground. The operational steps are repeated until the boiler is completely disassembled.
The boiler 10 to be disassembled usually has the superheaters 24 in the form of bending pipes therein, and the heat exchange pipes are provided in an inner casing of the boiler 10. Steam in the pipes is superheated to an extremely high temperature by the heat generated in the boiler 10. Therefore, it is necessary to start a disassembling operation after confirming that the temperature and pressure in the pipes.
In the method disclosed in Japanese Kokai Publication 11(2001)-270154, the entire weight of the boiler is supported by the jacks provided on the top girder of the boiler shed. Then, the boiler is gradually lowered in the state where the boiler is suspended from the top girder by use of the jacks, and the boiler is cut little by little. Such operation could be dangerous in some circumstances, sine the jacks are operated (lowered) while supporting the weight of the large portion of the top girder and the boiler.
Moreover, in accordance with the method of Japanese Kokai Publication 11(2001)-270154, it is necessary to completely disassemble the boiler in the first place, and that the boiler shed is disassembled from the top thereof, subsequently. Namely, the boiler and the boiler supporting structure are separately disassembled in the different steps. Thus, it takes a long time to perform both steps successively.
Moreover, it is necessary to carry out a pretreatment and aftertreatment each in the steps for disassembling the boiler and the step for disassembling the boiler supporting structure. Therefore, the method includes complicated procedures.
Furthermore, operations at a high place is required for the above discussed steps A to C for providing jacks, and for the step F for disassembling the boiler shed. Therefore, the method includes a dangerous disassembling step.
As to the method disclosed in Japanese Kokai Publication 2003-301617, a cut part of the boiler is conveyed by using the descent-ascent frame. Therefore, the method needs not suspend the entire boiler. In Japanese Kokai Publication 2003-301617, however, the ascent-descent stage has to be suspended from the jacks on the boiler shed. In a practical point of view, it is difficult to perform the method because the space around the boiler, particularly around the furnace unit which contains many attachments such as a control floor and piping that could be obstacles for the operation. Accordingly, it is very difficult to set the jacks on the top of the boiler shed so that a member for suspending the stage does not interfere with the attachment of the boiler.